


Who Put those Tables There?

by mathmusic8



Category: Redwall Series - Brian Jacques, Undertale (Video Game)
Genre: Because I literally know no one in both fandoms, Does this crossover even exist?, Gen, I guess it does now haha, I mean I have more ideas, Inter-dimensional travel, Not Beta Read, Obscure Crossover, One shot?, Poor Martin is reduced to normal mouse sounds, but I want to know if anyone's interested first, but they get through it
Language: English
Status: Completed
Published: 2020-07-30
Updated: 2020-07-30
Packaged: 2021-03-05 21:07:11
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,776
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/25601821
Author URL: https://archiveofourown.org/users/mathmusic8/pseuds/mathmusic8
Summary: Gaster and Sans finally open a portal to the outside world, but it doesn't go quite how they planned.
Comments: 2
Kudos: 8





	Who Put those Tables There?

**Author's Note:**

> This idea has been rolling around the back of my head for a few days now. I don't think this has really been done before, so we're in pretty new territory haha.

[“It worked! Oh stars, of heavens! Sans, it _worked_!!”] Gaster squealed.

Sans didn’t need to be told—he stood side by side with Gaster in front of a portal that showed rolling green hills, puffy white clouds against a deep blue canvas, and a yellow light brighter than Sans had ever seen in his life hanging in the ceiling—no, _sky_ , Sans corrected himself. It was a light in the _sky_. “is that the sun?” he whispered reverently.

Tears streamed freely down Gaster’s face. [“Yes! Yes, my dear boy! That’s the sun! The surface never looked so beautiful in all my life!”] He reached out as if to touch the very image, but he caught himself and sighed. [“We must be very careful. This is new territory, my boy—anything can happen!”]

Sans nodded dumbly. They stood there for several more minutes, knowing there was so much to do and to record and to test, but unable to tear themselves away from the sight of the wind rippling through the grass, rustling the dense group of trees at their right, and pushing the clouds across the sky. Even the sun moved, albeit slowly, and to Sans’s amazement, it seemed to burn even more brightly the lower it fell, bathing the scene in golden hues as it approached the distant horizon.

“i’ve seen pictures,” Sans finally said, still unable to bring his voice above a whisper. “seen movies and all that, but . . . .”

[“They don’t do it justice, do they?”] Gaster smirked.

Sans just shook his head. His shaking knees finally failed him, and he sank to the floor, sitting cross-legged. With his eyesockets stretched open wide and eyelights blown huge, he looked more like a child than Gaster had ever seen him.

Gaster grabbed his clipboard and sat beside him, taking notes at a furious pace but in reverential silence. The sun finally disappeared on the horizon, though its light remained for several minutes. Then, one by one, little specks of light began to appear in the ceiling—no, _sky_ , Sans forcibly reminded himself.

“that’s what a real star looks like?” Sans breathed. “i can’t believe it—i’m looking at a _real_ _star_!”

[“Technically, the sun is a star,”] Gaster stated, still scribbling away.

Sans just shook his head, never taking his sockets off of the portal for a moment. Being a skeleton, he had no need to blink, either, and he took full advantage of that fact.

Gaster’s bones started to ache from sitting on the floor, so after one last long look at the stars popping out by the dozens, he groaned and heaved himself to his feet. Sans started to follow, clearly reluctant, and Gaster set a hand on his shoulder. [“No, stay there. Tell me the moment anything changes with the portal. I’m going to check the machine.”]

Sans just nodded and resumed his staring at the beauty before him.

The novelty had finally worn off on Gaster, who spent the next several hours taking notes from the monitors and projecting how long the portal would last, the likelihood of it allowing matter to pass through it, and so on. The stars were bright against a black sky by the time Gaster was ready to perform his first test, and only then did Sans move.

[“Would you like to do the honors?”] Gaster asked, holding up a rope.

“sure,” Sans nodded. He stood up and took the long coil. Gaster checked their recording equipment one last time, then gave him the go-ahead.

Sans carefully dropped one end of the rope through the portal. There was a pop and sizzling sound, but when Sans drew the cord back into their dimension, it was still whole. They conducted similar tests the rest of the night, but when the telltale signs of dawn approached in the outside world, they called a halt and sat back down in front of the portal to watch. The portal was facing the wrong way to see the actual sun rising, of course, since they’d been able to watch the sunset, but its light still reached over the land. Birds started singing, and flowers bobbed on the hillside in a stiff morning breeze. The wind and temperature was still lost on the skeletons, since they did not seem to translate through the portal, but just the sight and sound of it all was intoxicating.

Sans had pulled more than one all-nighter recently, and it was all finally starting to catch up to him. He unconsciously slumped against his uncle’s shoulder, and Gaster smiled down at his nephew, noting that his eye sockets were blinking for the first time in nearly twelve hours, despite Sans’s best efforts to keep them open. [“The portal doesn’t seem to be going anywhere, my boy,”] Gaster laughed, slinging his arm around Sans’s shoulders. [“I expect we’ll see many more sunrises and sunsets, in time.”]

Sans hummed in agreement, and his eyesockets finally drifted closed. Gaster smiled and carried his nephew to the nearby couch, laying a blanket over him. He stayed awake for several more hours himself, but eventually, his head grew too heavy for him to keep up, and Gaster fell asleep on his desk, pen still in hand.

It was several days of intense testing later before Sans and Gaster were ready to try stepping through the portal themselves. Sans was decked out in a hazmat suit with a sturdy rope tied around his waist, and again, they started small. Sans lifted his boot and slowly inched it towards the portal with bated breath. Gaster watched unblinking as Sans’s boot stopped just at the moment of impact and hung there.

“there’s resistance,” Sans reported.

This was new. Based on every test they had conducted so far with both inanimate objects, plant life, magical items, and even a fly, there had been no indication of any resistance at all.

Sans announced, “i’m pushing harder,” and Gaster watched with wide eyesockets as Sans leaned more and more weight against his foot. His boot creased, as if he were holding his foot up against a wall, but it didn’t move forward.

They tried different spots of the portal, different amounts of force, even tried using their bare hands, thinking it might have been something about the suit, but it was all the same.

Repeating their previous tests came back with the same results as before—other objects and insects could pass through just fine, but the monsters themselves (and their attacks) were met by an invisible forcefield as steady as the cavern walls around them, as impenetrable as the barrier.

The barrier.

That was the conclusion they settled on. The magic that made up the barrier was strong enough to detect their portal and refused to allow them to escape.

Many tears were shed that day. Freedom was there— _right there_ —but they could do nothing. They had changed nothing. The experiment was a failure.

“unless a human comes by,” Sans realized, once their throes had started to diminish. “it just means we have one more entrance for them now, right? maybe we’ll get souls faster now.”

Gaster was less optimistic. [“How long have we had this open, two weeks? And we haven’t seen any sign of human activity whatsoever.”]

“still though,” Sans persisted, “an entrance is an entrance.”

[“We don’t even know if anything from that side _can_ pass through,”] Gaster protested. [“We’ve only pulled back what we’ve thrown out there.”]

“then let’s test it,” Sans said decisively.

So they did. They were able to collect dirt and grass samples, and they even tried to send a drone through, but it lost connection to the controller the moment it passed through the portal. Luckily they always tied a line to everything they sent out, so they were able to pull it back.

It took some time, but they were able to set up a rig with cables and wiring long enough to reach through the portal, and so they set up some simple cameras and motion sensors on the other side in the hope that someday, they’d be able to trap a human inside.

Time passed, as it always does. There was a scare at the Core which took Gaster several weeks to fully resolve, and Sans’s family started worrying about how much time he spent at the labs, so the portal studies started to take the backseat to the demands of everyday life.

It was about a month later that their motion sensor went off.

Gaster was eating lunch and taking notes on the machine at the time, concerned over the amount of power it took to run it and trying to determine if it had contributed to the latest scare at the Core. He didn’t notice the motion indicator beeping for at least a couple minutes, but once it did register, he startled so hard that his salad was knocked all over his notes, the bowl and several papers sliding against the floor, and Gaster’s chair was flung into its back as he dashed to the portal and the monitor screens set up around it.

It was a mouse.

Disappointed at first, Gaster released a breath he hadn’t realized he’d been holding, but then he gasped.

_Any_ living creature from the other side of the portal would be a fantastic study!

He needed to lure that mouse inside the portal.

Gaster cast about, looking for anything that could help him—cheese, did he have any cheese? The remains of his poor salad caught his eye, and Gaster quickly made a hook out of some wire, stabbed a piece of lettuce onto it, and tied it to a string. He tossed out his crude fishing line and watched the mouse startle backwards as the lettuce landed beside it.

For the first time, Gaster realized it was rather odd-looking for a mouse. It seemed to walk on its back legs just fine, and its fur was brown other than . . . _green_ across its top? There also seemed to be some kind of stick tied to its back, and Gaster wondered briefly who had tormented the poor thing—wait, had they put _clothes_ on it? That patch of green was almost like a shirt! Humans were prone to do odd things to their pets, but to do something to a wild animal was just uncalled for.

Positively giddy at the thought that humans might be close by, that this mouse could be the start of something bigger, Gaster waited with the upmost patience for the mouse to take the bait.

Except it didn’t.

It seemed to distrust the lettuce, but instead seemed to be fascinated by the long cables connecting Gaster’s sensory equipment to his lab. It followed the cables, first to the equipment, which it sniffed and pawed over for some time, and then it began stalking closer and closer to the portal.

Gaster stood stock still. All he needed was for the creature to step through to his side—even a paw or a whisker might be enough—and then he could grab it with his magic.

Just a few more steps now.

The mouse approached in a slow but steady step, eyes constantly shifting about in an intelligent manner.

Gaster was grateful he was a skeleton and could stop breathing and blinking by choice.

Two more steps.

One more.

There!

The mouse met no resistance as it slid into Gaster’s lab, still following the cables.

As soon as the creature’s tail had cleared the portal, Gaster turned it blue, effectually gluing it to the floor.

A cacophony of shrill squeaks and squawking exploded from the mouse, but Gaster made a quick bone cage around it and turned off his blue magic. He picked up the cage and carefully set it on his desk, The mouse within still laid flat on its belly, and seemed to be absolutely vibrating with fright.

[“I need to call Sans,”] Gaster realized, and at the sound of his voice, the poor mouse scrabbled to the far side of the cage, back on its hind paws now. Gaster picked up another lettuce leaf and pushed it into the cage while he dialed Sans’s number, and while the phone rang he got up to look for some kind of water dish.

Sans finally answered his phone. _“sup, doc?”_

[“Sans I caught a mouse!!”]

_“oh. uh, good job?”_ Sans said in a bemused voice. _“those are pretty rare down here, right?”_

[“No, Sans, I caught a mouse from the other side! From—from the other—”] Ah, there was a small plastic container by the sink that Sans had brought leftovers in once. Gaster quickly set about washing it, only half aware of how garbled his sentences were in the meantime. [“The sensor went off, but it’s not a human, but it’s all dressed up, so maybe a human will come looking for it! And it came through the—the, uh,”] why couldn’t he think if that word? [“the doorway just splendidly, all on its own, no resistance whatsoever! Though it didn’t seem to like lettuce—do you think you could bring some cheese, Sans?”]

“too late, sorry doc,” Sans said from directly behind him.

[“Yeeaarrgh!”] Gaster whirled around with the plastic container half full of soapy water still in hand. The sharp motion threw the water all across Sans’s face. [“Oh—oh—I’m terribly sorry, Sans, but you startled me!”]

Sans laughed and wiped suds out of his eyesockets. “’s all good doc, heh heh. so you got a mouse through the portal?”

Gaster smacked his forehead. [“Portal! That’s the word! Yes, yes, it’s over on my desk!”]

Gaster turned back to finished washing up the container, and Sans shuffled over to the bone cage, but then he grunted in surprise. “uh, doc? i think your mouse escaped.”

[“WHAT??!”] Gaster dropped the container in the sink and darted to Sans’s side. There was a clean mouse-sized hole cut straight out of the bone cage. [“How on earth . . . ? Quick, we have to find it!”]

No sooner said than done. A shrill squeaking erupted at the portal itself.

Before Gaster or Sans could even think to use blue magic, they watched mouse hurl itself at the portal, but instead of passing through, its body flattened against an invisible wall. The mouse slid down the barrier and tried again and again, even grabbed the stick off its back and—oh, that was _not_ a stick! It was a covering of some kind, a covering for a bright, shiny piece of metal that the mouse held in its front paws like—like a sword! It had a _sword_! The mouse hacked desperately at the invisible wall, and the sword fell through to the other side without resistance, but the wall prevented the mouse’s paws from following, causing the creature to drop the weapon to the other side.

The creature’s squeaks reached a new pitch as it began to well and truly panic, and at that point Gaster finally remembered his magic and pinned it down with blue. The mouse fell flat on its face, paws spread out and body trembling in distress. Gaster made a new bone cage around it and passed it to Sans.

[“Here, keep an eye on it,”] he said.

Sans jerked out of his shock and took the cage back to the desk, setting it beside the old one. Gaster found a pen nearby and used it to reach through the portal and flick the tiny sword back to their side of reality. Gingerly picking the weapon up by the tiny hilt between two fingers, he brought it over to the table.

[“It must be pretty sharp to be able to cut through bone,”] he remarked, turning to test it on the old cage. Indeed, the blade sliced through the bones easily.

“someone wants his knife back,” Sans remarked, nodding down to the new cage.

The mouse was standing on its hind feet, reaching between the bone bars of its prison towards his sword, chattering up a storm.

Gaster was fascinated.

[“It acts like it’s sapient,”] he said. [“I thought a human had dressed it up, but do you suppose . . . ?”]

“the barrier didn’t let it back through,” Sans pointed out, rubbing his chin. “you think it’s gotta soul?”

[“Let’s find out.”] Gaster held a hand over the cage and turned the mouse blue again, dropping the creature on its back to the floor of the cage. A quick adjustment to the magic in the bones formed hinges allowed Gaster to open the top of the cage, and he and Sans peered inside.

A tiny blue spade, a proper monster soul shape, glowed above the mouse’s heaving chest as the creature continued to scold them. Gaster released his magic, just to make sure.

The mouse’s soul did not fade into white, however.

No, it’s soul’s natural color was scarlet red.

Gaster slammed the top of the cage closed again, and Sans’s bones started to rattle.

“the mouse is determined,” Sans whispered.

[“The mouse is determined,”] Gaster agreed. He set his palms flat against the table to steady himself, but all the excitement had taken its toll. [“I . . . I need to sit down.”]

Sans carried the cage in one hand and supported his uncle by the elbow, walking him to the couch. They sat there together in silence for a long while—even the mouse had quieted down, sulking in the corner of its cage.

Curiosity eventually got the better of Sans. He noticed the destroyed salad for the first time and saw pieces of shredded carrot littered around with the lettuce. He used blue to float some over and poked one long piece of carrot through the cage.

The mouse was a blur, attacking the thin piece of carrot with its tiny claws and teeth as if it were a snake. Sans huffed a small, startled laugh, grateful he hadn’t stuck his finger inside like he’d been tempted to. The mouse seemed to recoil in surprise for a moment, perhaps realizing this new object was actually food, and then backing away from it. Sans just pushed the carrot into the cage, adding a few more for good measure.

[“I was cleaning a little plastic container for its water dish when you came in,”] Gaster said dully, without otherwise moving.

Sans nodded and set the cage on the couch for a moment. He retrieved the container, filled it with water and approached the cage. Then he had a second thought. “don’t want it to spill on the couch,” he said, picking up the cage and bringing it back to the desk. He turned the mouse blue with just enough pressure to keep its footpaws on the floor of the cage before he opened the top and set the water inside. The mouse looked from the water to the carrot sticks for a moment, then back up at Sans’s face. It’s shoulders seemed to slump, and gave a soft squeak.

“you’re welcome,” Sans answered. He wasn’t sure that’s what the mouse was trying to say, but he was fairly certain now that it was actually speaking a real language. A pity its words didn’t appear over its head, like with other monsters. Perhaps it had something to do with the fact that it was from a difference dimension.

The air conditioning kicked on. Sans wouldn’t have even noticed it—being in Hotland, the air conditioning was nearly always running in the lab—but the mouse seemed to shiver and wrapped its arms around itself.

Sans looked up at Gaster, but the old doctor had stretched himself out on the couch with his arm thrown over his eyesockets.

“hm. looks like it’s just you and me for the moment, pal,” Sans murmured. He looked around for something small and soft enough to act as a blanket for the poor creature, and then he had an idea. “be back in a sec,” he whispered.

Sans stepped out of the portal room and into his bedroom with some spacetime magic. He was quick to find what he needed and came back with an armful of socks. Again lightly weighing the mouse down with blue magic, Sans opened the top and lined the cage with socks of all sizes and colors. The mouse picked up one within its reach and ran its paws over the fine material. It blinked up at Sans with what he thought might have been awe.

“really wish i could talk to you, little pal,” Sans sighed. He closed the top of the cage and sat at the desk, scratching his head at the puzzle.

The mouse began organizing its gifts, pushing the water container into one corner, piling the shredded carrots beside it, and then laying out the socks over the other half of the cage. It kept an extra soft blue sock on top of the heap and laid on top of it experimentally. Finding it satisfactory, the mouse unbuckled the scabbard from its back, set it carefully aside, and rolled up the sleeves of its tunic. Sans watched in fasciation as the mouse scrubbed its paws in the water dish and picked up a carrot stick, breaking it in half to a more manageable size. Only then did it sit cross-legged and start to eat.

This was a sophisticated person.

Sophisticated people often—not always, but often—had a written language.

Sans got to work.

Finding the paper was easy—rip any piece of paper into sections and there you have it. The pencil had to be whittled down to a more miniscule size, but it was manageable. Using his own pencil, Sans wrote as tiny as possible, still filling half the sheet, _Can you write?_ Even if the mouse had a written language, could read it, and could write it—there was no guarantee of any of the above—there was next to no chance that it could read what Sans had written. Still, it was worth a shot. Sans slid the paper and tiny pencil into the cage. The mouse seemed to understand what it was, picking it up and looking at the letters, but by the way it turned the paper this way and that, it couldn’t tell which way was up or down.

So it did have a language, but their languages were not the same. Good to know.

To Sans’s relief, the mouse picked up the pencil stub and wrote a response. When it was done, it pushed the page back through the bars, washed its paws again, and returned to its food.

As expected, the markings weren’t any language Sans knew, but he was a skeleton, and skeletons by nature grew up decoding things. He just needed enough samples.

It took some time, and the two drew crude pictures to communicate in the meanwhile, but in the end Sans was confident that he had found the key to the mouse’s written language. Curiously enough, it mirrored the Common language perfectly, letter for letter. To Sans, it was just a different font.

Thus they began a written exchange.

Sans learned that the mouse’s name was Martin of Redwall, son of Luke the Warrior, and he was more or less the head honcho of a group of woodland creatures who had freed themselves from evil wildcats. Martin had been on a summer stroll when he stumbled onto Gaster’s device, and, being a curious soul, had followed the cables straight into the lab. His sword was made from a fallen star by a great badgerlord, and Martin promised not to harm anyone if it was returned to him. Unsure of what Gaster wanted him to do, Sans made no promises and quickly changed the subject.

Martin’s mode of speech—or, well, writing—was formal and archaic, but Sans discovered he also had a quick sense of humor, and soon they were exchanging puns and riddles like they were old friends.

[“What are you giggling at over there?”] Gaster groaned, lifting his arm to glare at Sans.

“have a nice nap?” Sans asked as he passed his latest joke through to Martin.

Martin took the paper, but he looked to Gaster anxiously when the old skeleton sat up with another groan.

[“How long was I asleep?”] Gaster asked.

“long enough for me to learn a new font,” Sans shrugged. “wanna see?”

Gaster sighed and stood up. [“What do you mean, ‘new font’? There aren’t ‘new fonts’, just uncommon—”] He saw the scraps of paper laying in neat rows on his desk and glanced over the decoded messages. [“It can communicate,”] he gasped, suddenly wide awake.

“sure can,” Sans said, his eyes crinkled in a happy smile. “’s gotta name, too. Martin. He said he’s grateful for the food but wants out of the cage and wants his sword back, please and thank you. He promised he doesn’t want to hurt us, too.”

Gaster pulled up a second chair and looked at the mouse. Martin looked back, cocking his head curiously. [“Does he know he can’t go back?”]

“i mean, we haven’t talked about it, but . . . .” Sans shrugged. They had both seen the mouse throw himself against the barrier with no effect.

[“Tell him explicitly please,”] Gaster said. [“I don’t want a determined mouse running around destroying things with that little bone-slicing toothpick of his thinking he can break the barrier.”]

Sans nodded and composed a short explanation of their situation, requesting Martin to please be careful around their expensive equipment. Martin agreed.

[“Alright, here’s hoping we don’t regret this immediately,”] Gaster sighed, but he obligingly dissolved the cage.

Martin squeaked in surprised as he dropped to the desk, but he recovered quickly and lost no time recovering his sword. To everyone’s relief, he simply put it back in its sheath and buckled it over his back again, and then he brought his pencil stub to a full sheet of paper that Sans had in reserve and began writing.

_What is to become of me?_ Sans translated.

[“Ask him if he’d be willing to allow me to perform some experiments with him,”] Gaster requested.

Sans gave him a startled look.

Gaster spread his hands wide. [“What? It’s not like I can do anything without his consent. He’s a person, same as you and me.”]

“but what do you want to do?” Sans prompted.

[“Oh, I don’t know—well, we need to test his determination eventually, obviously, but there’s plenty of simple things to start with. A scan would be a good idea. So would magic tests, and maybe a soul study. Has he noticed anything odd since coming here? If he is from an entirely different dimension, something as basic as gravity might be different! Does he have super strength? Can he fly? Just think of the possibilities, Sans!”]

Sans shook his head. “okay, okay, i get the picture, you nerd.” He thought for a moment, then wrote, _we’d like to know more about you and where you come from. would that be alright?_

Martin replied, _Yes, I will continue to answer your questions. I also have many questions._ After a pause, he seemed to take in a deep breath and continued, _Can I really never go back?_

Sans read the mouse’s words aloud and looked up to Gaster. “i mean, in theory, if we break the barrier, the portal should work both ways, right?”

Gaster nodded. [“Explain it all to him.”] He took another sheet of paper and started writing down his ideas for questions, tests, and theories about Martin’s dimension, allowing Sans plenty of time to explain the barrier to Martin.

When Gaster chanced a glance, he blinked in surprise—was the mouse _crying_? He seemed to be wiping his eyes, sniffing, and he even pulled out an itty bitty handkerchief to wipe his snout.

Gaster forced himself to look away. Of course the mouse was crying—Gaster had cried, too, when he was first locked inside the barrier. Cried, and screamed, and raged, and cried some more. At least they had hope now that they had a few human souls.

Martin agreed to do some basic tests, but Gaster decided they could wait until things had settled down, encouraging the mouse to take his time to process this in whatever way he needed to. They set up more food and separate water containers for him to drink and wash in. For the first time in quite a while, both Sans and Gaster left the labs at the end of a normal workday, allowing Martin some privacy.

Before heading home however, Sans and Gaster went to Grillby’s, and they talked in low tones over their drinks long into the night.

**Author's Note:**

> Thanks for reading! Please let me know if you find any typos, or if you have any comments in general!
> 
> The essence of Martin’s squeaks (Hopefully they aren’t necessary to explain, but just in case):   
> When he’s first captured: “Arrgh! I—can’t—move! What is this foul magic?”  
> In the cage, quietly, to himself, “didn’t even take my sword, the puddinghead.”  
> Back at the portal: “Why—can’t—I—get—through?? Redawaaaaaalll! NO! My sword!! All is lost!”  
> When Gaster has his sword. “Return my weapon and release me at once, giant foulbeast! You cannot keep a free creature here!”  
> When Sans gives him water, “Thank you.”


End file.
